routing: add capacity to special edges

Having a capacity available is important for liquidity estimation.

* We add a dummy capacity to hop hints. Hop hints specify neither the
  capacity nor a maxHTLC size, which is why we assume the channel to
  have a high capacity relative to the amount we send.

* We add a capacity to local edges. These channels should always have a
  capacity associated with them, even in the neutrino case (a fallback
  to maxHTLC is not necessary). This is just for completeness, as the
  probability calculation for local channels is done separately.

routing/unified_edges.go

@@ -218,7 +218,10 @@ func (u *edgeUnifier) getEdgeLocal(amt lnwire.MilliSatoshi,
 		maxBandwidth = bandwidth
 
 		// Update best edge.
-		bestEdge = &unifiedEdge{policy: edge.policy}
+		bestEdge = &unifiedEdge{
+			policy:   edge.policy,
+			capacity: edge.capacity,
+		}
 	}
 
 	return bestEdge